Update on anti theft fault message

[Jelevents]

Hi still having problems with anti theft message coming up on dash, however I've found if I turn on ignition on then very slightly turn the key but not quite enough to engage the starter the triangle with ! light goes out and car starts fine.
So far I've replaced the transponder ring and ignition switch and tried the BSI reset, neither made any difference, this morning would not start at all with either key so I disconnected battery left it all day reconnected still get the message but now starts again with both keys??????
So I'm thinking perhaps a new key would be the answer, but I don't have the pin code and main dealer or local specialist are booked up till mid October.
Is anyone on here able to plug in to diagnose fault codes I'm based in Bedfordshire but happy to travel a reasonable distance to find the answer?

Update.......I've now changed the com2000 unit on the steering column, made no difference I obtained the pin code took to a citroen specialist he found fault code F4BA but could not clear it, tried recoding the keys no luck then wouldn't start so disconnected battery started ok, but he had no idea what the fault is.
Currently not starting unless battery disconnected and left for while, anybody got any idea/ advise?
Car has done 177,000 miles and has noisy camshaft chain, egr n/w and front suspension lower arm bushes will need doing b4 next mot so I'm thinking it's time to sell it for spares or repair and try to source a replacement?????

[Jelevents]

Hi still have problems I've removed the engine ECU checked all connections no water ingress, removed Bsi/fusebox again all checked out ok, still starting intermittent and when it starts both keys work but still has dash message anti theft fault.
Any ideas it's driving me mad, I've bought a Peugeot partner now to enable me to work but would like to get the Citroen going so I can sell it cheap or otherwise it'll be scrapped which is a shame apart from starting issue it's a decent vehicle.

[Sol]

The BSI is a full-on computer so you would never be able to tell if it has a fault on the microelectronics inside, by looking at it. Thats where I think the issue is, since the BSI decides if the rolling security code between the key transponder and itself, is valid.

Sometimes the van is happy with the keys, other times not, points to an intermittent immobiliser issue and that's built into the BSI circuit board. I don't see you getting to the bottom of this easily. You've swapped out all of the simple culprits except the BSI itself, and that last part is not easy to do.

It's possible a matching set of keys, ECU and BSI off eBay might work, but every chance the config is different and other issues crop up.

Plus on nearly 200k miles with a noisy tensioner (it's a timing belt, not a chain) then it's whether you feel it's worth spending a chunk of money on it as it stands to fix the anti fault message, when you will have a timing belt, water pump and suspension work, to get it fully roadworthy again?